Microwave heating material set inside hydrogen making reactor is heated with microwave radiation penetrating the wall of the reactor in a sealed environment separated from outer air, so as to reach temperature for the gaseous hydrocarbon to be decomposed into hydrogen and carbon black. While maintaining the temperature of the microwave heating material inside the reactor, preheated, depurated andpressurized hydrocarbon gas flow is introduced and made to contact with the microwave heating material fully for reaction decomposing into hydrogen and carbon black. The reaction product is separatedto obtain hydrogen product and carbon black product. The production process may be performed continuously, and the microwave heating material includes both carbon black material and non-carbon black material.